White light emitting diode (LED) based solid-state lighting is commanding much attention worldwide for its promise of energy savings compared to incandescent and even compact fluorescent lighting. The energy efficiency, longevity, and material usage in manufacture are all attributes that favor white LED technology, yet technical problems persist. The predominant white LED technology involves the employment of high quantum efficiency (η≧60%) blue InGaN quantum well (QW) LEDs and the down conversion of blue radiation to yellow/green and red for white light generation. Y3Al5O12:Ce3+ and Eu2+ doped nitridosilicates have been coated onto the InGaN QW LED as yellow/green and red phosphors such that trichromatic “cold white light” is produced by mixing red, yellow, green, and blue emission in the LED output.
There are, nevertheless, a number of limits on the performance of those white LEDs due to the phosphor conversion scheme employed. Since the existing red, yellow, and green phosphors have different chemical compositions, it is difficult to control the granule size and to mix and deposit uniform multi-phosphor films. Also, the different aging behavior of the multiple phosphor species often makes the device performance unstable in terms of the overall wavelength output. Current white LED lamps also manifest this phosphor aging differential as a shorter than desirable lamp operation lifetime. A more fundamental limit on the efficiency of the phosphor conversion white LEDs, however, lies in the multi-step “down conversion” scheme: high energy, blue photons produced by InGaN QW LEDs have to be absorbed by the phosphors first, and then, via impurity-level assisted transitions, are converted to low energy, long wavelength photons with a one-to-one correspondence. This process loses a significant portion of the photon energy to lattice vibrations (heat) in the phosphor media as a non-radiative conversion and is also limited by electron system crossings between singlet and triplet quantum states. The energy loss in the down-conversion process will, by itself, set the ultimate quantum efficiency of white LEDs below 65%.
Colloidal compound quantum dots (QDs) have been introduced to the white LED technology as a new family of phosphor materials with many superior properties. Due to strong quantum confinement, semiconductor QDs, such as core/shell CdSe/(Zn,Cd)S QDs, are characterized by sharp exciton absorption spectral features, extremely high luminescence efficiency (˜90-95%), and size tunable emission color spanning the entire visible spectrum. QDs of the same chemical composition and different size can therefore be employed to provide multiple spectral components in white LED output, with improved color quality and aging performance. The most significant potential of QD phosphors lies in the recent discovery that there exists a path for indirect injection of electron hole pairs into QDs (for radiative recombination and thus band edge emission from QDs) by noncontact, nonradiative energy transfer from a proximal InGaN quantum well (QW).(1) The direct, non-radiative energy transfer path is considered to be the consequence of dipole-dipole interactions associated with QW-QD coupling and the extremely fast intraband relaxation in colloidal QDs (subpicosecond time scales). As indirect injection of electron hole pairs is fundamentally different from the traditional multi-step “down conversion” fluorescence scheme described previously and operative in traditional phosphors and removes several of the intermediate steps involved in color conversion, this approach has the prospect of eliminating energy losses associated with the steps and increasing the fundamental limit of efficiency.
As promising as the indirect injection of electron hole pairs into QD approach appears to be, it has to-date met with limit success. In one report, Chen et al. has demonstrated white LEDs by housing an. InGaN blue LED chip in a silicon resin doped with green and red emitting CdSe/CdS. Since the QDs were physically separated from the emissive QWs in the LED chip, no QW-QD coupling was possible. A low efficiency of 7.2 lm/W was recorded. (2).
In another study, Achermann et al. observed high efficiency color conversion in an electrically pumped light emitting diode (LED) using non-radiative energy transfer between an InGaN/GaN QW and a monolayer of CdSe/ZnS QDs.(3) Spectroscopic measurements revealed that 13% of the radiative power of the QW was transferred almost loss free to red emission from the QDs when the QW and QDs were located in close proximity to each other. The viability of this work was limited in producing an actual LED device owing to the difficulty in resolving the inherent conflict between the need for close proximity of the QW and QDs and also the need for a sufficiently thick electrical contact layer with a low resistance for LED operation. In addition, the energy fraction (13%) channeled between the QW and QDs is still too low for viable device formation to leverage the benefits of white LEDS relative to conventional lighting devices. These experimental devices with weak QD phosphor emission compared to the bright blue radiation from the InGaN QW remain impractical for usage.
Thus, there exists a need for quantum dots with varied emission colors coupled to an LED emitter that promotes efficient nonradiative energy transfer therebetween to achieve a practical white LED with low energy consumption. These also exists a need for a QW-QD white LED that is compatible with the presence of a bulk electrical contact layer.